Melt butter in a saucepan over medium-low heat; cook and stir onion in hot butter until soft and caramelized, 30 to 60 minutes.,Heat olive oil in a skillet over medium-high heat; cook and stir mushrooms until browned and tender yet crisp, about 10 minutes. Season with salt and pepper.,Preheat oven to 400 degrees F (200 degrees C).,Roll puff pastry out on a lightly-floured work surface. Cut out 5-inch circles using a round cutter and place on a baking sheet.,Bake in the preheated oven until lightly browned and puffed, about 12 minutes. Transfer puffs to a wire rack to cool completely.,______,Bake in the oven until warmed through, about 5 minutes.